The ultraviolet colour of globular clusters in M31: a core density effect?

TL;DR

This study reveals a correlation between core density and ultraviolet colours in M31 globular clusters, suggesting dense environments promote the formation of extreme-HB stars, with LMXBs contributing minimally to FUV emission.

Contribution

It demonstrates a significant relationship between cluster core density and UV colour, linking stellar density to the production of extreme-HB stars in globular clusters.

Findings

Dense clusters have bluer UV colours indicating more EHB stars.

The formation of EHB stars is likely enhanced by dynamical interactions in dense environments.

LMXBs contribute less than 10% to the FUV luminosity in clusters with blue-HB stars.

Abstract

We investigate the effect of stellar density on the ultraviolet (UV) emission from M31's globular clusters (GCs). Published far-UV (FUV) and near-UV (NUV) colours from Galaxy Evolution and Explorer (GALEX) observations are used as a probe into the temperature of the horizontal branch (HB) stars in these clusters. From these data, we demonstrate a significant relationship between the core density of a cluster and its FUV-NUV colour, with dense clusters having bluer ultraviolet colours. These results are consistent with a population of (FUV bright) extreme-HB (EHB) stars, the production of which is related to the stellar density in the clusters. Such a relationship may be expected if the formation of EHB stars is enhanced in dense clusters due to dynamical interactions. We also consider the contribution of low mass X-ray binaries (LMXBs) to the integrated FUV luminosity of a cluster. We note that two of the three metal rich clusters, identified by Rey et al. 2007 as having a FUV excess, are known to host LMXBs in outburst. Considering the FUV luminosity of Galactic LMXBs, we suggest that a single LMXB is unlikely to produce more than 10% of the observed FUV luminosity of clusters that contain a significant population of blue-HB stars.